Radiance: How does the focus of a "spotlight" work?
Found some explanation here:


  The focal length produces only a subtle difference as it shifts the
  effective position of the source behind the actual emitting surface. A
  real spotlight has the effect of changing its falloff to being
  something less than r^-2 (where r is the distance to the emitter) in
  this way. The actual falloff is closer to (r+e)^-2, where e is the
  focal length of the spotlight's optics.  It is a minor concern, I
  admit.